Abstract Teaching is a highly stressful profession and toddler teachers report substantially higher stress levels than teachers of older children, resulting in teacher turnover rates 4 times higher than those of elementary staff and jeopardizing their abilities to foster children?s early development. Early Head Start (EHS) teachers, serving more than more than 160,000 at-risk infants and toddlers annually, may be at the greatest risk for stress. As many as 25% of Head Start staff have significant prior trauma exposure. In our pilot with EHS teachers, 31% reported 4 or more traumatic events. Prior exposure to trauma is robustly associated with greater vulnerability to mental health problems in adulthood, including stress. Hence, there is a great need to better understand teachers? stress, including coping and sources of stress that may account for variability in stress and associations between trauma exposure and stress. Existing research has only examined stress for short periods of time or via general surveys, limiting understanding of teachers? real-time stress over the school year. We propose to use an innovative ecological momentary assessment approach to examine stress (intensity of stress) and coping (mindfulness and social support) in 100 EHS toddler teachers using an existing sample. We are currently funded to implement and evaluate a 13-week professional development (PD) intervention (Hearts and Minds on Babies; HMB). Leveraging the HMB study, this R21 will describe the stress trajectory of toddler teachers by extending stress monitoring of teachers to include brief bi-weekly assessments over the school year. We will also examine coping and sources of stress that account for expected variability in stress. We will assess teachers? prior trauma exposure and its associations with baseline stress and stress trajectories. We also have a unique opportunity to examine differences in teacher stress by HMB intervention group assignment. Aims:(1) Describe the stress trajectory for toddler teachers and test sources of stress and coping as accounting for variability in stress; (2) Examine the effect of prior exposure to trauma on the initial stress level; (3) Identify differences in stress for toddler teachers by PD intervention condition. Approach: We will collect 2X weekly data over the school year (totaling 40 weeks Sept. to June; 80 measurement occasions) on coping and stress (via a smartphone app). Trauma exposure (Aim 2) and intervention group status (Aim 3) data will be provided from the HMB study. Scientific Premise: Chronic stress places heavy demands on teachers? abilities to cope, and jeopardizes their abilities to engage in high quality practices that support children?s development. In order to develop effective PD, there is a great need to better understand teachers? stress, including sources of stress and coping that may account for variability in stress, and the ways in which trauma exposure relates to stress. Impact: Results will elucidate real-time stress in toddler teachers, identify characteristics accounting for variability in stress, and show how PD intervention may impact stress. Findings will inform PD efforts for teachers serving our youngest and most vulnerable children.